Method for producing pulp and products from high silica content agricultural waste materials

ABSTRACT

A method of producing a high quality paper pulp slurry from high silica content lignocellulosic agricultural waste materials is provided. The method includes combining the agricultural waste materials with a chemical composition to form a mixture, sautéing the mixture, and grinding the sautéed mixture to provide the pulp slurry. The method results in greater than 55 percent solids recovery in the pulp slurry relative to the starting lignocellulosic agricultural waste materials, and removes substantially none of the lignin from the agricultural waste materials.

This application claims priority from Chinese Patent Application No. 02 2 33439.4 filed May 9, 2002. BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to producing useful products from agricultural waste stocks. More particularly, it relates to producing high quality paper pulp from annually renewable agricultural residues or waste materials such as hay and straw, having high silica content.

[0003] 2. Description of Related Art

[0004] Conventionally, paper pulp is produced via mechanical or chemical pulping processes using wood chips as the starting material. In mechanical pulping processes, the wood chips typically are not chemically treated (except to add copious amounts of water), and are ground into small pieces to form the pulp. Water is added to the ground small pieces in order to provide the pulp with the desired water content, typically above 90-95 percent by weight. Pulp produced from wood chips via the conventional mechanical pulping process generally has high recovery on a solid weight basis (e.g. greater than 80 or 90 percent) of the original wood chips. In other words, about 100 lbs of wood chips pulped via the conventional mechanical pulping process typically produce greater than 80 or 90 lbs (dry basis) of pulp.

[0005] In conventional chemical pulping processes, the wood chips are treated chemically to remove lignin, a sticky glue like substance that binds cellulose together in the living plants (e.g. trees) from which the chips were harvested. Generally, the process involves treating the chips in a highly alkaline solution (e.g. 10-30 weight percent NaOH based on dry weight of the chips) and cooking the chips in this solution with high pressure steam in order to remove or extract the lignin from the chips. This highly caustic mixture of alkali and steam at high temperature and pressure cannot be safely discharged to the environment. Therefore, it is recycled in the process and used to treat subsequent batches of wood chips in order to delignify them.

[0006] Following chemical treatment, the resulting delignified chips must be rinsed, often several times, in order to wash away excess or residual alkali. Any alkali residue in the chips that is not washed away prior to producing the finished pulp will not only have negative environmental impacts, but also could damage pulp or paper processing equipment which uses the finished pulp to make paper products. In addition, residual alkali represents a health and safety hazard to consumers of finished paper products.

[0007] Once the lignin has been removed from the wood chips, the chips must be smashed and combined with water in order to achieve the appropriate consistency and water content for the finished pulp (again typically greater than 90-95 weight percent). The above-described rinsing step or steps can also be combined with this step. Alternatively, the chips can be mixed and smashed into pulp consistency during the alkaline delignification step described above. The finished pulp is then used to produce paper products as known in the art via conventional techniques.

[0008] The conventional chemical pulping process for pulping wood chips as described above generally result in only about 50-55% recovery on a solid weight basis of the original wood chips. In other words, about 100 lbs of wood chips would produce about 50-55 lbs (dry basis) of pulp. The remaining 45-50 weight percent of the wood chips is discarded as waste. This waste must be treated to reduce or remove residual alkali before being discharged to the environment.

[0009] In addition to the drawbacks noted in the preceding paragraph for chemical pulping of wood chips, the production of pulp from wood chips, whether by the conventional mechanical or chemical process described above, contributes significantly to global deforestation. The demand for paper products is growing faster than the trees that are planted to replace those harvested. Therefore, alternative sources of raw material for making paper pulp (i.e. other than wood) are highly desirable.

[0010] Lignocellulosic materials other than wood can be used to produce pulp. For example, pulp can be produced from agricultural materials such as corn stalks and husks, rice straw, wheat straw, bagasse and other grasses, and other cereal materials such as grain materials and grain stalks. Unfortunately, neither the conventional chemical nor mechanical pulping process described above is suitable for producing pulp from the vast majority of these materials (including corn stalks, rice straw, wheat straw, etc.). First, the conventional mechanical pulping process used for pulping wood chips results in grinding these materials to a fine powder, not useful for pulp. This is because the cellulosic fibers in these materials are not nearly as strong as those found in woods for which the mechanical pulping processes were designed. The only exception is sugar cane residue, whose fibers are sufficiently strong to withstand pulping using the conventional mechanical pulping process without being ground to powder.

[0011] Second, with regard to the conventional chemical pulping process, the high silica content of such materials as stalks and husks, straws, grasses and cereals make them highly unsuitable for this process. Wheat straw contains 4-10% silica by weight as small crystals embedded in the straw; rice straw has an even higher silica content of 9-18% by weight; other cereals such as barley, oat and rye straw have 1-6% silica by weight (all on a dry solids basis). Wood on the other hand has a silica content of less than 1% by weight (dry solids basis), making wood suitable for the conventional chemical pulping process.

[0012] Silica is incompatible with the highly caustic alkaline environment used in the chemical pulping process. In the chemical pulping process described above, silica dissolves in the alkali solution during the cooking step and remains in the cooking liquor (black liquor). The black liquor contains inorganic material used as the cooking chemicals, such as hydroxides to produce the alkalinity, and both inorganic and organic materials removed from the fibrous raw materials (wood or nonwood) during cooking. All modern pulp mills include a chemical recovery process to treat the black liquor and recover the pulping chemicals. In addition, organic non-cellulose material from the black liquor is recovered (up to 50% of the original material before cooking) to generate energy for the pulp mill and to recycle process water. In conventional chemical pulping processes, for every one ton of pulp that is produced, approximately seven tons of black liquor are produced which must be dealt with; i.e. recycled and/or abated.

[0013] A problem with cereal straw and other high silica content materials is that the high silica content causes many problems in the chemical recovery process for the black liquor, such as scaling (coating equipment with a glass like substance) which reduces the efficiency of some equipment due to increased viscosity, making it difficult or impossible to pump the black liquor through portions of the recovery system. The viscosity can be increased to so great an extent as to effectively plug up the recovery equipment. These problems make chemical recovery difficult, less efficient and more costly compared to recovery for black liquor from wood which has less than 1% silica by weight. If the silica content of the cereal straw is not too high (less than 5-6%) it is possible to include a modified wood-based recovery system, but at higher capital and operating costs. But for higher silica content, especially for rice straw, there is no presently available solution for producing pulp that includes a waste recovery or recycling mechanism. Without waste recovery, a pulping process is extremely polluting and less energy efficient, which result in added operational costs for pollution abatement and control and higher energy costs.

[0014] In particular, the silica in straws and cereals makes recycling of the highly alkaline delignification solutions impossible due to complex silicate-alkali chemical reactions. Instead, they must be treated batchwise to neutral pH prior to discharging to the environment. In addition, a fresh alkali solution must be prepared for each new batch of the raw materials, further compounding the complexity and the expense of making pulp. For these reasons, high silica content materials such as cereals and grasses generally are not used to produce paper pulp to any significant degree, despite their virtually inexhaustible supply. Furthermore, even when they are used (necessitating batchwise waste stream abatement and substantially preventing any waste recovery), cereals and grasses generally yield only about a 40% solids weight recovery via conventional chemical pulping techniques; 10-15% lower on average than for chemical pulping of wood chips, and 40-50% lower than for mechanical pulping of wood chips. All these factors combine to make high silica content lignocellulosics, such as cereals and grasses, highly undesirable and underutilized in the production of paper pulp, despite their vast supply. For this reason, there is currently no commercially or economically acceptable method for producing high quality paper pulp from annual or renewable materials. Rather, present methods require wood chips from trees that do not grow at nearly or even at a fraction of the rate at which they must be cut down to provide the chips.

[0015] There is a need in the art for a method of producing paper pulp from high silica content lignocellulosic raw materials, such as cereals, straws and grasses, which are renewable on an annual basis, that can be operated in a continuous process, and which does not result in the generation of significant or substantial wastes. Preferably, such a process will result in greater than 55% weight recovery of the starting raw materials.

SUMMARY OF THE INVENTION

[0016] A method for producing a pulp slurry is provided having the steps of: a) providing a lignocellulosic material having a silica content of at least 1% by weight on a dry solids basis; b) combining the lignocellulosic material with a chemical composition to form a mixture, the chemical composition having 0.4-4.8 weight percent base reckoned as alkali hydroxide, 0.2-4 weight percent bleaching agent, 0.1-10 weight percent alkali metal silicate, and 0.001-10 weight percent chelating agent, all on a dry solids basis relative to the lignocellulosic material; c) sautéing the mixture; and d) comminuting the sautéed mixture to produce the pulp slurry.

[0017] A further method for producing a pulp slurry is provided having the steps of: a) providing a lignocellulosic material having an initial lignin content and a silica content of at least 1% by weight on a dry solids basis; b) combining the lignocellulosic material with a chemical composition to form a mixture; c) sautéing the mixture; and d) comminuting the sautéed mixture to produce the pulp slurry, the pulp slurry having substantially all of the initial lignin content of the lignocellulosic material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a side view of an apparatus for producing paper pulp according to the invention, including a cooker unit and a milling unit.

[0019]FIG. 2 is a side close-up view, partially broken away, of the cooker unit of FIG. 1 for producing paper pulp according to the invention.

[0020]FIG. 3 is a side close-up view, partially in section, of the milling unit of FIG. 1 for producing paper pulp according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0021] As used herein, when a preferred range such as 5-25 (or 5 to 25) is given, this means preferably at least 5 and, separately and independently, preferably not more than 25.

[0022] The invention includes a method for producing pulp from high silica content agricultural waste materials. Agricultural waste materials that can be used to produce pulp according to the invention include straws (rice straw, corn straw, barley straw, oat straw, and other straws), husks (e.g. corn husks), bagasse, grasses, hay, as well as other lignocellulosic stalks or material stocks that have a high silica content, e.g. greater than 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, percent by weight on a dry solids basis.

[0023] The high silica content agricultural waste materials are mixed and treated with a chemical composition according to the invention and sautéed at elevated temperature as will be more fully described below. The resulting sautéed mixture is then ground and combined with water to produce a pulp slurry as will also be described. The chemical composition preferably includes the components listed below in table 1. In table 1, any preferred or less preferred or more preferred weight percent or weight percent range of any component can be combined with any preferred or less preferred or more preferred weight percent or weight percent range of any of the other components; it is not required or necessary that all or any of the weight percents or ranges for all components come from the same column. All values in table 1 are percentages by weight of the high silica content waste materials (bone dry basis) with which the composition is mixed. For example, a weight percent of 4 listed in table 1 means that for 100 lbs of agricultural waste material, 4 lbs of the component listed in table 1 are provided in the composition that is mixed with the agricultural waste material. TABLE 1 Composition for treating high silica content lignocellulosics to produce pulp Component Most Preferred Less Preferred Less Preferred Base (reckoned as 1.6-3.4 1-4 0.4-4.8 alkali hydroxide) 1.8-3.2 1.2-3.8 0.6-4.4 2-3 1.4-3.6 0.8-4.2 Bleaching agent 0.8-1.2 0.5-1.5 0.2-4   0.9-1.1 0.6-1.4 0.3-2   1 0.7-1.3 0.4-1.8 Alkali metal silicate 2.6-4.4 1.5-5.5 0.1-10  2.8-4.2 2-5 0.5-6.5 3-4 2.5-4.5 1-6 Chelating agent 0.02-0.06 0.006-1.5  0.001-10   0.03-0.05 0.008-1   0.002-4   0.04 0.01-0.7  0.004-2  

[0024] The above composition is preferably combined or mixed with the high silica content lignocellulosic agricultural waste materials just prior to being fed to a cooker unit to be sautéed as will be further described. The agricultural waste materials typically contain about 10 weight percent water with the balance (˜90 weight percent) being solid material. However, depending on environmental factors such as rain fall, humidity, harvesting practices, etc., the waste materials may have a water content from 1-90 weight percent or more. It is important to consider the relative moisture content of the waste materials when measuring the amounts of the components listed in table 1, which are based on the bone dry weight of the waste materials, excluding moisture content. Preferably, the waste materials are supplied having a moisture content of about 10, less preferably 8-12, less preferably 6-14, less preferably 4-16, less preferably 2-20, less preferably 1-30, weight percent, balance solids.

[0025] The base component from table 1 is provided to induce a softening effect on the high silica containing agricultural waste materials, such as rice stalks or hay. These stalks are generally very rigid, tough fibrous materials and the base component is supplied to make them more pliable and subject to more effective sautéing as will be explained below. The values listed in table 1 are for bases or hydroxides reckoned as alkali hydroxide. So, for example, if some other hydroxide (such as Ca(OH)₂) or basic (i.e. pH raising) agent is used instead of an alkali hydroxide, then one must calculate or determine the amount of the substituted hydroxide or basic agent required to supply an equivalent amount of hydroxide or hydroxide reducing power as the amount reckoned as alkali hydroxide listed in table 1. In the event another hydroxide or basic agent is used, this calculated amount should be substituted for the corresponding value in table 1 when preparing the composition according to the invention. Most preferably the base component is sodium hydroxide. Less preferably, other hydroxides, such as other alkali hydroxides, alkali earth hydroxides, heavy metal hydroxides, or other metallic hydroxides, can be used. Less preferably, other bases and basic agents can be used in appropriate amounts to supply equivalent hydroxide reducing power as the amount of base reckoned as alkali hydroxide listed in table 1.

[0026] The bleaching agent is provided to prevent the sautéed waste materials from becoming blackened or darkened in appearance as a result of sautéing, which is typical of sautéed cellulosic materials. The bleaching agent provides the sautéed waste materials with a light or tan coloring, that does not appear blacked or charred as a result of being sautéed, e.g. in a cooker unit. The bleaching agent is preferably a peroxide bleaching agent, preferably hydrogen peroxide. Less preferably, other peroxides can be used. Still less preferably, other non-peroxide bleaching agents, such as chlorinated bleaching agents can be used.

[0027] The alkali metal silicate component is provided to stabilize the peroxide agent described in the preceding paragraph, which otherwise may have a tendency to dissociate due to the relatively weak peroxide bond and the highly reactive oxygen atoms which are bound via the peroxide bond. Preferably, the alkali metal silicate is sodium silicate. Sodium silicate functions as a rigidizer in the finished pulp product. In addition, sodium silicate is abundantly and inexpensively available. Perhaps most importantly, unlike most silicates, sodium silicate is water soluble and hence is effectively solvated in the water supplied by the moisture content of the waste materials during the sautéing step as explained below.

[0028] The chelating agent can be an inorganic or organic chelating agent, or a mixture of these. Preferably, the chelating agent is inorganic, such as an inorganic salt, and preferably is selected from alkali sulfates, alkali phosphates and alkali carbonates, more preferably from alkali earth sulfates. Most preferably, the chelating agent is magnesium sulfate. Less preferably, suitable organic chelating agents include EDTA, NTA, and DTPA. The chelating agent is supplied in a very low amount to bind heavy metal impurities found in the agricultural waste materials. Magnesium sulfate is most preferred because, in addition to binding heavy metal impurities, magnesium sulfate acts as a further stabilizing agent for the peroxide component described above. The inventors herein have also found that the addition of magnesium sulfate to the composition for treating high silica content agricultural waste materials to produce pulp results in a surprising and significant reduction in the presence of impurities, surprisingly higher uniformity in appearance and color, and a minimization of dark spots in the finished pulp product.

[0029] In practice the above-described chemical composition is combined with the high silica content agricultural waste materials, and together they are continuously fed into a cooker unit where the composition is mixed with the agricultural waste materials and becomes solvated in the water from the moisture content of the agricultural waste materials. In one preferred embodiment, no additional or excess water is supplied to the mixture in the cooker unit, besides that which is supplied as the moisture content of the waste material. Alternatively and preferably, additional water can be added to bring the total moisture content within the cooker unit (relative to the agricultural waste materials+water) to 10-90, preferably 20-80, preferably 30-70, preferably 40-60, preferably about 50, percent by weight. In the cooker unit, the chemical composition-agricultural waste material mixture is sautéed at a temperature of 100-300, preferably 120-280, preferably 140-260, preferably 160-240, preferably 180-220, preferably about 200, ° C., for a period of time long enough to completely sauté the agricultural waste material solids in the water supplied from their own moisture content and additional supplied water, if any, in which the chemical composition described above is dissolved or solvated. Most preferably, in the cooker unit the mixture of agricultural waste materials, water and added chemical composition being sautéed comprises about 50 weight percent water. After sautéing, this mixture is discharged from the cooker unit into a milling unit where the mixture is ground up to produce a slurry of ground solid pieces suspended or mixed in a light colored aqueous liquid. At this stage, additional water is added to bring the total solids content of the slurry to 0.02-5, preferably 0.05-2, most preferably about 0.1-0.2 weight percent. This slurry is discharged from the milling unit as a pulp slurry, and is used to produce paper products via conventional techniques.

[0030] Now with reference to FIGS. 1-3, a preferred embodiment for a method of producing pulp according to the invention will now be explained. A pulping apparatus 100 including a cooker unit 21 and a milling unit 22 is provided. The cooker unit 21 includes a straight tube 51 having an inlet 7 at one end and an outlet 16 at the other end. A conveying device, such as an auger 25 or other rotating or helical conveying device is provided within the straight tube 51 to propel the waste materials from the inlet 7 toward the outlet 16. The auger 25 has a shaft 11 and a helical blade 13. The shaft 11 extends axially outside of the straight tube 51 and is fixed at either end to bearing seats 5 and 15 respectively. The axially extending portions of the shaft 11 are fitted with sealing devices 6 and 14 respectively in order to ensure a fluid tight seal at the point where the shaft 11 translates through the outer wall of the straight tube 51 at either end thereof. The shaft 11 is driven in rotation via a drive motor, with or without gears, belts, or other transmission means in a conventional manner. In a preferred embodiment, (seen in FIG. 2), the shaft 11 is connected via a speed reducer 3 to a belt roller 2, and the belt roller 2 is driven in rotation via a belt connected to a motor (not shown). The shaft 11 is connected to the speed reducer via a shaft coupling 4. The rotational speed of the shaft 11 is regulated via the speed reducer 3 as known in the art. The direction of rotation of the shaft 11 is selected so that the corresponding rotation of the helical blade 13 results in waste materials deposited into the straight tube 51 through inlet 7 being conveyed toward the outlet 16.

[0031] The straight tube 51 includes an inner, agricultural waste material passage 52 where the auger 25 is located, and an outer annular passage or heating jacket 9. In the preferred embodiment, a heat transfer fluid, preferably a liquid, most preferably an organic liquid or oil, is provided in the heating jacket 9, and cycled therethrough co-current to the direction of travel of the agricultural waste materials within the waste material passage 52. Alternatively, the heat transfer fluid can be provided in a countercurrent flow direction relative to the agricultural waste materials. The heating jacket 9 is preferably part of a closed system with spent or cooled oil exiting from valve assembly 12, being reheated via conventional means (not shown in the drawings) and then reintroduced through inlet 8. The heating jacket 9 is preferably provided with a helical vane 10 so that the heat transfer fluid or oil travels in a co-current (or countercurrent) helical path relative to the straight tube 51, ensuring maximum fluid-surface contact and effective heat exchange.

[0032] In the illustrated embodiment, the outlet 16 of the straight tube 51 is arranged to deliver sautéed agricultural wastes into the milling unit 22. Though an elbow 24 is shown between the straight tube 51 and the outlet 16, the elbow should not be considered an essential feature of the pulping apparatus 100; e.g. a straight or other suitable path for delivering materials from the straight tube 51 to the milling unit 22 may be employed.

[0033] The milling unit 22 includes an upper grinding plate 45 and a lower grinding plate 47. The lower grinding plate 47 is driven in rotation via drive shaft 32 which is in turn driven by a driving motor. In a preferred embodiment, as shown in FIG. 3, the drive shaft 32 can be coupled to a belt roller 31 which is driven via one or a plurality of belts by a drive motor 46. Alternatively, other driving or transmission means conventional in the art for rotating the shaft 32, e.g., gears, speed reducing machinery, etc. can be used. The upper grinding plate 45 is either fixed, or is rotated in a direction opposite the direction of rotation of the lower grinding plate 47. Alternatively, one of the grinding plates 45,47 is movable or shearable relative to the other such that a shearing grinding force can be provided between adjacent surfaces of the grinding plates.

[0034] A feed tube 43 is connected to the outlet 16 of the cooker unit 21, to deliver the sautéed agricultural wastes to the grinding plates 45, 47. In the illustrated embodiment, the milling unit has a casing that includes an underbody casing 48 that is butt-jointed with an upper casing 44. The moving machinery of the milling unit 22 is provided within this casing. The drive shaft 32 is fixedly connected to the lower grinding plate 47 and a lower tray 33. The upper grinding plate 45 is located directly above and adjacent the lower grinding plate 47, and has an associate upper tray 36 above the upper grinding plate 45. The upper grinding plate 45 is spaced some distance from the upper tray 36 to leave a gap therebetween, and the lower grinding plate 47 is correspondingly spaced some distance from the lower tray 33 to leave a corresponding gap therebetween. The feed tube 43 passes through the upper casing 44 and the upper tray 36 which are preferably secured via bolts 38. The nose or delivery end of the feed tube 43 is positioned adjacent or within a central hole of the upper grinding plate 45. The respective gaps between the upper grinding plate 45 and upper tray 36 and the lower grinding plate 47 and lower tray 33 are fluidly connected with an outlet port 37 in the casing of the milling unit 22. A locking device is provided for securing the upper casing 44 to the underbody casing 48. The locking device includes a pair of adjusting plates 41 and 42 and a pair of locking plates 39 and 40. The adjusting plates and locking plates are screwed or secured around an upward projecting section of the feed tube 43 to secure the upper casing 44.

[0035] In operation, the method of the invention preferably proceeds as follows. High silica content agricultural waste materials are first harvested or obtained via conventional means. These agricultural waste materials typically are comprised of about 90 weight percent solids and about 10 weight percent water. These waste materials (solids and water content) are combined with the chemical composition described above and the resulting mixture is provided into the inlet 7 of the apparatus 100. Preferably, additional water is added to bring the total water content of the mixture in the waste material passage 52 to 40-60, most preferably to about 50, percent by weight. From the inlet 7, the agricultural waste materials-chemical composition mixture is conveyed through the waste material passage 52 by operation of the auger 25. As it is conveyed, the mixture is sautéed at a temperature of 100-300, most preferably about 200, ° C. via heat transfer from heating oil passing through the heating jacket 9. In the waste material passage 52, the high silica content waste materials are sautéed in their own juices, supplied by their own water content, additional supplied water, if any, and the dissolved chemical composition. The chemical composition treats the agricultural waste materials as described above while they are being sautéed; e.g. preventing blackening or darkening of the agricultural waste materials, binding heavy metals and other impurities, etc.

[0036] The rotational speed of the auger 25 is adjusted to provide a suitable residence time within the cooker unit 21 (waste material passage 52) for sautéing of the agricultural waste materials. After passing through the agricultural waste material passage 52, the fully sautéed agricultural waste materials are delivered into the feed tube 43 of the milling unit 22 via elbow 24. The grinding plates 45,47 comminute or grind up the or sautéed agricultural waste materials while at the same time (or upon entering the milling unit 22) water is added to these agricultural waste materials. The ground up mixture may circulate between the grinding plates 45,47 and in the gaps between respective upper and lower grinding plates and trays as described above, until the mixture ultimately is eluted from the milling unit 22 via outlet port 37 as an aqueous pulp liquid or slurry. Upon exiting the milling unit through the port 37, the pulp has a solids content of 0.02-5, preferably 0.05-2, most preferably about 0.1-0.2 weight percent as mentioned above. In addition, the pulp includes greater than 55, preferably at least 60, preferably at least 70, preferably at least 80 preferably at least 90, preferably at least 93, preferably at least 95, preferably at least 97, preferably at least 99, percent of the solid content of the original agricultural waste materials, representing greater than 55, preferably at least a 60, 70, 80, 90, 93, 95, 97, or 99 percent solids recovery relative to the starting agricultural waste materials. This is a dramatic and highly significant and surprising advance over the state of the art which uses wood chips as the starting material for producing pulp, where solids recovers typically do not exceed 50-55 percent.

[0037] An important feature of the present invention is that the chemical composition does not remove lignin from the waste materials. In fact, by the present invention, lignin is not removed at all from the agricultural waste materials before producing the pulp. Instead, the pulp is produced having substantially all or 100% of the initial lignin content of the starting lignocellulosic waste materials, and yet is still very useful and effective at producing high quality, very strong paper products such as for packaging applications, a highly surprising and unexpected result. Without wishing to be bound to a particular theory, it is believed that so significant and surprising a solids recovery for pulp produced according to the present invention as stated in the preceding paragraph is due at least in part to the fact that lignin is not removed from the waste materials to produce the pulp. Therefore, not only does the lignin not need to be abated or cleaned up as a waste product, but the lignin mass remains with the agricultural waste materials and is transferred into the finished pulp, raising the solids recovery.

[0038] Another distinction from conventional chemical pulping techniques is that in the present invention, the agricultural waste materials are sautéed in a relatively small amount of water. In the conventional process, the material being pulped is not sautéed, but rather is fully cooked in a continuous or substantially continuous bulk liquid phase of water present in an amount many times that of the waste materials by weight, analogous to submersion in boiling water as one might boil an egg. In the present invention, by providing, in the most preferred embodiment, only about 50 weight percent water in the cooker unit, the agricultural waste materials are not submersed in a continuous water phase and cooked as by boiling; instead the waste materials are sautéed in a manner more akin to a Chinese style of cooking or stir-fry. The inventors herein have found that this method of sautéing the waste materials yields a finished pulp material that has surprisingly good and strong qualities, particularly in the making and fabrication of highly durable and strong packaging materials, having impact and shear strength characteristics similar or superior to conventional cardboards, yet having surface smoothness and writability characteristics similar to writing paper.

[0039] The pulp produced according to the present invention has been found to be very useful in making very strong, light colored paper products, e.g. packaging materials, having strength comparable to or better than conventional cardboards used in packaging. That the pulp produced according to the invention would be so strong and light colored compared to conventional cardboards was also a surprising and unexpected result.

[0040] In sum, the invention includes a method for producing pulp from high silica content agricultural waste materials (lignocellulosics), that results in greater than 55 percent solids recovery, and that does not remove any or substantially any of the lignin from the starting agricultural waste materials, yet results in a light colored pulp that is useful to make very strong paper products having greater or comparable strength relative to conventional cardboards used in packaging. The resulting paper products (from pulp produced according to the invention) can be produced having a very smooth surface, much smoother than conventional cardboards.

[0041] From the above description it will be apparent that, contrary to conventional pulp-making methods, which utilize trees/wood as the raw materials, the invented method can be performed on a continuous basis at steady-state to produce paper pulp, as opposed to a batch process. The raw materials for the present invention are not wood chips, but high silica content lignocellulosic agricultural waste materials such as cereals and straws, which are annually renewable sources that would otherwise be burned as waste. Further, the method according to the invention can be performed entirely at atmospheric pressure, and does not need any special high-pressure handling equipment or piping, for example to accommodate high pressure steam. In addition, the method for producing pulp from high silica content lignocellulosic waste materials according to the invention produces no or substantially no waste (e.g. no solid, liquid or gaseous waste), and thereby presents no or substantially no pollution control problems or issues of any kind; unlike the conventional chemical pulping processes that use wood chips as the raw material, which produce about seven tons of black liquor waste for each ton (dry basis) of pulp produced.

[0042] Although the hereinabove described embodiments of the invention constitute the preferred embodiments, it should be understood that modifications can be made thereto without departing from the scope of the invention as set forth in the appended claims. 

What is claimed is:
 1. A method for producing a pulp slurry comprising the steps of: a) providing a lignocellulosic material having a silica content of at least 1% by weight on a dry solids basis; b) combining said lignocellulosic material with a chemical composition to form a mixture, said chemical composition comprising 0.4-4.8 weight percent base reckoned as alkali hydroxide, 0.2-4 weight percent bleaching agent, 0.1-10 weight percent alkali metal silicate, and 0.001-10 weight percent chelating agent, all on a dry solids basis relative to said lignocellulosic material; c) sautéing said mixture; and d) comminuting said sautéed mixture to produce said pulp slurry.
 2. A method according to claim 1, further comprising adding water to said mixture after said sautéing step to provide said pulp slurry with a total solids content of 0.02-5 weight percent.
 3. A method according to claim 1, said bleaching agent being hydrogen peroxide.
 4. A method according to claim 1, said alkali metal silicate being sodium silicate.
 5. A method according to claim 1, said mixture being sautéed at a temperature of 100-300° C.
 6. A method according to claim 1, said mixture being sautéed at a temperature of about 200° C.
 7. A method according to claim 1, said pulp slurry having greater than 55 percent solids recovery relative to said lignocellulosic material.
 8. A method according to claim 1, said pulp slurry having at least a 60 percent solids recovery relative to said lignocellulosic material.
 9. A method according to claim 1, said lignocellulosic material having an initial lignin content, wherein lignin is not substantially removed from said lignocellulosic material to produce said pulp slurry, said pulp slurry comprising substantially all of said initial lignin content of said lignocellulosic material.
 10. A method according to claim 1, being performed at substantially atmospheric pressure.
 11. A method according to claim 1, said lignocellulosic material having a moisture content of 1-30 weight percent, balance solids, said chemical composition being solvated in water supplied from said moisture content of said lignocellulosic material.
 12. A method according to claim 1, wherein no additional water is supplied to said mixture during or prior to said sautéing step besides the water supplied from said moisture content of said lignocellulosic material.
 13. A method according to claim 1, said lignocellulosic material having a moisture content of about 10 weight percent, balance solids.
 14. A method according to claim 1, further comprising adding water to said mixture during or prior to said sautéing step to provide a total moisture content, relative to said lignocellulosic material, of about 50 percent by weight.
 15. A method according to claim 1, performed on a continuous basis at steady-state.
 16. A method according to claim 1, further comprising: a) providing a pulping apparatus, said pulping apparatus comprising a cooker unit and a milling unit, said cooker unit including an agricultural waste material passage having an inlet and an outlet, and a conveying device located within said agricultural waste material passage, said milling unit having a first grinding plate and a second grinding plate; b) providing said mixture into said inlet of said agricultural waste material passage; c) conveying said mixture through said agricultural waste material passage by operation of said conveying device; d) sautéing said mixture at a temperature of 100-300 degrees Celsius as said mixture is conveyed through said agricultural waste material passage; and e) delivering said mixture into said milling unit, said comminuting step being performed by grinding between said first and second grinding plates to produce said pulp slurry.
 17. A method according to claim 16, said lignocellulosic material being sautéed in juices supplied from a moisture content of said lignocellulosic material, said chemical composition being dissolved or solvated at least in part in water from said moisture content.
 18. A method according to claim 16, further comprising moving or shearing at least one of said first and second grinding plates relative to the other grinding plate to provide a shearing grinding force between adjacent surfaces of said first and second grinding plates to comminute said mixture to produce said pulp slurry.
 19. A method according to claim 1, said chelating agent being selected from the group consisting of alkali earth sulfates.
 20. A method according to claim 1, said chelating agent being magnesium sulfate.
 21. A method for producing a pulp slurry comprising the steps of: a) providing a lignocellulosic material having an initial lignin content and a silica content of at least 1% by weight on a dry solids basis; b) combining said lignocellulosic material with a chemical composition to form a mixture; c) sautéing said mixture; and d) comminuting said sautéed mixture to produce said pulp slurry, said pulp slurry comprising substantially all of said initial lignin content of said lignocellulosic material.
 22. A method according to claim 21, said pulp slurry having greater than 55 percent solids recovery relative to said lignocellulosic material.
 23. A method according to claim 21, further comprising adding water to said mixture after said sautéing step to provide said pulp slurry with a total solids content of 0.02-5 weight percent.
 24. A method according to claim 21, further comprising adding water to said mixture during or prior to said sautéing step to provide a total moisture content, relative to said lignocellulosic material, of 30-70 percent by weight. 